1. Field of the Invention
The invention is directed to a method for the transmission of voice information in ATM cells.
In transmission systems that are operated according to asynchronous transfer mode (ATM Mode), the messages to be transmitted are transmitted in message cells having a fixed length. The message cells comprise an information part as well as a cell header. The information part serves for the acceptance of the messages, whereas the cell header preceding the information part carries information about the allocation of a message cell to a virtual connection. The cell header usually exhibits a length of 5 octets and the information part usually exhibits a length of 48 octets.
There is a general aim to realize the universal communication network for voice, text, data and image (ISDN) on the basis of the asynchronous transfer mode of public and private networks. However, the problem arises in that, the voice information transmitted in the previous transmission technique (pulse code modulation technique PCM) must be linked into message cells and vice versa. Such a conversionxe2x80x94dependent on configuration and linking of the ATM communication networkxe2x80x94must be implemented numerous times under certain circumstances.
The linking of voice information into ATM cells is currently provided according to the standardizations of the ITU in the AAL 1 adaption method. Specific properties of the voice information are taken into consideration and specific, one-byte wide information, also called AAL 1 header, are deposited in a predetermined field of the information part of an ATM cell. This means that the information part only has 47 octets available for transmission of payload data (voice information) instead of 48 octets. FIG. 1 shows the relevant fields of the AAL1 standard which are described below.
Further adaption methods are provided for the linking and transmission of other information data. For example, methods for linking and transmission of these other information data are employed in AAL5. These other information data can, for example, be signaling data, computer data or other payload data. Since the network cards integrated in current terminal equipment fashioned as personal computers only support the AAL5 standard, the aim is to transmit voice information with the assistance of the AAL5 standard.
Currently, there are essentially two proposals in the ATM forum as to how voice information can be linked and transmitted in ATM cells upon employment of the AAL5 standard (ATM Form/96-0950, ATM-Forum/960955).
It is provided therein that octets 1 through 40 or, respectively, 1 through 32 of the information part be filled with voice information leaving the remaining octets for other fields, such as trailer data. A check sum and the length of the data packets to be transmitted are deposited in the fields for the trailer data, along with other user-specific information.
These proposals do not consider the specific conditions of voice transmissions, in contrast to the transmission of voice information according to the AAL1 standard. For example, when one of the ATM cells is lost during the transmission procedure, the loss cannot be identified without further efforts according to the AAL5 standard.
Since this standard is only useful for data transmission of a general type, all data are collected as necessary in a cyclical time grid in the terminal equipment and are subjected to an extensive test. This procedure is not critical for computer data transmissions since a time linking in the sense of an on-line transmission is not established here. In this respect, it can be accepted without further efforts to set a permanently prescribed time span for this review test and to wait and to re-request lost data as needed from the transmission source later.
With voice information, by contrast, there is the requirement that the transmitted data are immediately exchanged between a sender and receiver. The permanently prescribed time span that is allowed for computer data transmissions is too lengthy for voice information transmissions. When, voice information are to be transmitted according to the AAL5 standard, then the methods described in the prior art cannot be applied since the loss of ATM cells here can only be identified at the receiver side with extensive testing, resulting in a reduced voice quality.
2. Description of the Prior Art
U.S. Pat. No. 5,446,726 discloses a device for error identification and error correction for information that are transmitted according to an asynchronous transfer mode (ATM), handling and possibly correcting overload situations. Transmission of voice information is not addressed in detail.
European Patent Application EP 0 725 506 discloses a device and a method for the transmission of multimedia data, proposing that data that are defined according to the MPEG2 layer be transmitted in the AAL5 ATM adaption layer. Transmission of voice information is not addressed in detail here either.
An object of this invention is to provide a method for efficiently transmitting voice information in ATM cells according to a standard that is not based on specific conditions for voice transmissions, but rather based on the features by the adaptation of various fields in the ATM Cell to accommodate information pertaining to voice data.
What is particularly advantageous about the invention is that specific information with respect to the transmission of voice information are inserted into a part of the octets of the information part, so that the fields of the AAL1 header employed in the AAL1 standard are interworked. Hence the advantages according to this standard are linked to those of the AAL5 standard, such as the determination of a loss of an ATM cell.
A further object of the invention is to provide a method whereby the loss of ATM cells carrying voice information can be identified at the receiver side based on the criterion of the information with respect to specific conditions of the transmission of voice information (i.e., specific voice data condition information), and incorrectly inserted ATM cells can be identified.
A further object of the invention is to provide a method whereby the information with respect to specific conditions of the transmission of voice information are inserted into octets 37 through 40.
A further object of the invention is to provide a method whereby, when given the loss of at least one ATM cell carrying voice information occurs, ATM cells are additionally formed and supplied to the receiving equipment instead. It is thus assured that, given loss of ATM cells, just as many ATM cells are inserted as ATM cells are lost.
A further object of the invention is to provide a method whereby the information with respect to specific conditions of the transmission of voice information comprise a pointer that indicates the starting address of the data block to be transmitted in the ATM cell. This involves the advantage that data transfer for Nxc3x9764 kbits/s structured in the sense of the AAL1 standard can be implemented.
A further object of the invention is to provide a method whereby the information with respect to specific conditions of the transmission of voice information potentially contain information wherein time information, such as local time of day, are deposited.